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The Debate: Is Pluto A Planet, Asteroid Or Comet?

KNOXVILLE, Tenn. — Since its discovery six decades ago, school children have been told Pluto is a planet in our solar system. Now, there’s disagreement.

Dr. Mark Littmann, University of Tennessee-Knoxville astronomy and journalism professor and author of an award-winning book on the solar system, takes issue with several astronomers who recently said Pluto should not be considered a planet.

“Pluto is a planet — by orbit and size,” he contends.

Pluto is too large to be an asteroid because it “has about three times more mass than all the asteroids in the solar system put together,” Littmann says.

Secondly, he says, Pluto is in the wrong place to be an asteroid. The vast majority of asteroids orbit the Sun between Mars and Jupiter.

“Pluto’s distance from the Sun places it not far from the inner edge of the vast cloud of perhaps a trillion comets that surrounds the planetary portion of the solar system,” he says.

“Pluto’s icy composition and its orbit … are somewhat comet-like. But, again, Pluto is far too large (to be a comet).”

Littmann says there is nothing to be gained by excluding Pluto from planethood.

“Size is not the decisive factor,” he says. “Mercury is smaller than one moon of Jupiter, yet no one is crying for Mercury to be deleted from the list of planets.”

Pluto cannot be classified an asteroid because “there is no asteroid belt known beyond Neptune for which Pluto can serve as a charter member,” Littmann suggests.

“To designate Pluto as an asteroid or a comet is to ensure that it will virtually drop from treatment in classrooms and schoolbooks, and a new generation of children will grow up thinking of Pluto as a ‘mistake,’ if they have any awareness of Pluto at all,” he says.

Pluto is best understood and taught in astronomy courses as a “probable example of a large outer solar system planetesimal, the kind of object formed from comet-like bodies…at the beginning of our solar system,” Littmann said.

“Pluto and its moon Charon have potential value to science, not as representatives of a class of asteroids or comets, but as large, nearly pristine planetesimals — the objects from which the gas…planets and their moons are made.